1. Field of the Invention
The present invention relates to a structure for the connection of a plurality of electronic dievices to an ARINC (Aeronautical Radio, Inc.) type bus.
It can be applied especially but not exclusively to electronic devices installed on aerodynes that communicate with one another by means of one or more buses that meet the ARINC 629 standard.
2. Description of the Prior Art
According to this standard, as shown in FIG. 1, the electronic devices Ei are connected to a main cable 1, or aircraft cable, by means of a stub or &lt;&lt;dual descent&gt;&gt;cable 13 that is connected to the main cable 1 by means of a simultaneously two-way coupler 5 of the non-intrusive, inductive type. This coupler provides for the conversion between the voltage dipoles circulating in the stub and the current dipoles circulating in the main cable. This topology allows for distances in the range of 100 meters between the devices and enables the connection of a maximum of 120 devices.
The main cable 1 constituting the bus is used in multiplexed multi-transmitter mode so that the information elements circulating in the bus are transmitted by only one device at a time and then distributed to the other devices connected to the bus. The transmitted information elements are monitored in real time by the transmitting device which does so by means of the information elements that are restored through the connection of this device to the bus.
The stub 13 is formed by a dual twisted pair, i.e. one pair for the transmission channel 4 and one pair for the reception channel 3 thus enabling simultaneous transmission and reception on the stub and hence real-time monitoring of transmission. It is controlled in the device Ei by an SIM 629 serial interface module providing for the physical interface between the stub and the TC 629 function or terminal controller function of the device. This function controls the transmission and reception of messages of the sub-system that uses the equipment.
The fact is that the field in which collisions occur, namely the zone in which signals coming from different devices may be present simultaneously, corresponds to the entire main cable. Major distances and therefore major disparities (in terms of attenuation, parasitic phenomena and propagation time) on the signals therefore have to be processed in the SIMs or serial interface modules of each device. Furthermore, this detection must be done as swiftly as possible and with the best possible coverage so as to make the communications system rapidly available for the other devices.
The result thereof is that the main cable must have low attenuation, controlled impedance with low tolerance (2%) throughout its length as well as very precise matching (4%), given that all the passive elements of the physical layer should not induce false physical information in the form of parasitic reflections which will then have to be processed by the devices.
Furthermore, the SIM of each device must be capable of monitoring and adapting to variable levels of signals in reception, with the need to distinguish between, on the one hand, accurate physical information during transmission, hence high-level information, and physical information that is transmitted remotely and is therefore at a level that is attenuated and, on the other hand, information coming from instances of collision.
In the current approach, the elements of the physical layer, namely the main cable, the SIM of each equipment as well as the stub and the connection coupler of each device are very costly. This is because of the very narrow tolerance required for the physical characteristics of the passive components and because of the need to ascertain that the signals received come within the complex-shaped templates that differ according to whether the operation relates to reception for the control of a local transmission or to the reception of a remote transmission.
Furthermore, the detection of collision for which the SIM is responsible is done in a complex way by means of a very precise analysis of the amplitude of the signal received at specific instants, the width of the pulses and the intervals between the pulses.